Re: [syzbot] [kernel?] INFO: task hung in restrict_one_thread_callback

[Hillf Danton]

On Fri, Feb 20, 2026 at 03:11:21AM -0800
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    635c467cc14e Add linux-next specific files for 20260213
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1452f6e6580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=61690c38d1398936
> dashboard link: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16e41c02580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15813652580000

#syz test  upstream master

--- x/security/landlock/tsync.c
+++ y/security/landlock/tsync.c
@@ -432,12 +432,17 @@ static void cancel_tsync_works(struct ts
 int landlock_restrict_sibling_threads(const struct cred *old_cred,
 				      const struct cred *new_cred)
 {
+	static int concur = 0;
 	int err;
 	struct tsync_shared_context shared_ctx;
 	struct tsync_works works = {};
 	size_t newly_discovered_threads;
 	bool found_more_threads;
 
+	if (concur++) {
+		concur--;
+		return -EBUSY;
+	}
 	atomic_set(&shared_ctx.preparation_error, 0);
 	init_completion(&shared_ctx.all_prepared);
 	init_completion(&shared_ctx.ready_to_commit);
@@ -556,6 +561,7 @@ int landlock_restrict_sibling_threads(co
 		wait_for_completion(&shared_ctx.all_finished);
 
 	tsync_works_release(&works);
+	concur--;
 
 	return atomic_read(&shared_ctx.preparation_error);
 }
--